Download Opal as a Java jar file (why Java? read this):

Opal.jar (you may need to right-click, then choose "save link as")

No need to unpack or make anything.
Source code is included in the jar.
You must have Java version ≥ 5 (aka v1.5) for Opal versions 0.9.x and up (Java ≥ 1.4 for earlier Opal versions).
Read the documentation page for further instructions.

Current Version

Opal.1.0.3.jar (02/25/09)
Fixes error when guide tree is provided. Previous version would iterate, building a new tree (and crash along the way).

Previous Versions (placed here for the sake of posterity; no reason to use them)

Opal.1.0.2.jar (02/25/09)
Correctly handles windows newline characters in fasta files.

Opal.1.0.1.jar (02/13/09)
Reduced peak memory consumption by roughly 40%.

Opal.1.0.0.jar (01/06/09)
The first official release, coincident with it's release in Mesquite. Several interface and logging changes to accomodate inclusion in Mesquite.

Opal.0.9.3.jar (11/11/08)
Includes support for nucleotide sequences

Opal.0.9.2.jar (10/31/08)
Opal.0.9.1.jar (10/30/08)
Opal.0.3.7.jar (10/17/07)
Opal.0.3.6.jar (09/06/07)
Opal.0.3.5.jar (08/31/07)

opal_0.2.0.tar.gz (05/08/07)
opal_0.2.0.zip

To unpackage the files, run
   gzip -dc opal_0.2.0.tar.gz | tar -xvf -
or
   unzip opal_0.2.0.zip

To compile and run opal, do the following:

cd alignalign
make all
cd ../opal
make all
./Opal -f your_file